6
SENSING SYSTEMS

Introduction

To support the development of smart drilling systems, incremental advances that can come from a systems approach to the entire rock breaking and removal system must be recognized (Onoe and others, 1991). Yet, the intelligence behind the smart drilling system lies in the sensing systems to be incorporated in each drilling system component. Sensing systems include devices that measure and analyze data, interpret results, and activate other systems in response to the interpreted results. Functions of sensing systems include measurements of the drill or tunnel borer, measurements of the geologic formation, and measurements of the interaction between drill bit and rock, as well as positioning and telemetry. Sensing systems analyze measurements by human intervention, as well as by completely autonomous expert-driven systems.

Sensing on drilling systems has rapidly evolved over the last 20 years. Advances in the oil and gas drilling area, discussed here, generally pertain to other drilling technologies as well. Simple sensing of the rate of rotation, the rate of penetration, the torque, and the weight on the drill string, all measured at the surface, has gradually been enhanced to include several measurements of formation properties, in addition to downhole analysis of drilling parameters. These measurements are commonly made 30 to 50 ft behind the drill bit in special sections of the drill pipe. The data are analyzed by downhole computers, and because the drill pipe sections do not form a usable continuous electrical circuit, the final results are transmitted to the surface by pressure pulses in the mud column.

The array of formation measurements that is available while drilling is now approaching the set of standard wireline data that are collected after the drill string is removed, and this trend will probably continue (Cantrell



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6 SENSING SYSTEMS Introduction To support the development of smart drilling systems, incremental advances that can come from a systems approach to the entire rock breaking and removal system must be recognized (Onoe and others, 1991). Yet, the intelligence behind the smart drilling system lies in the sensing systems to be incorporated in each drilling system component. Sensing systems include devices that measure and analyze data, interpret results, and activate other systems in response to the interpreted results. Functions of sensing systems include measurements of the drill or tunnel borer, measurements of the geologic formation, and measurements of the interaction between drill bit and rock, as well as positioning and telemetry. Sensing systems analyze measurements by human intervention, as well as by completely autonomous expert-driven systems. Sensing on drilling systems has rapidly evolved over the last 20 years. Advances in the oil and gas drilling area, discussed here, generally pertain to other drilling technologies as well. Simple sensing of the rate of rotation, the rate of penetration, the torque, and the weight on the drill string, all measured at the surface, has gradually been enhanced to include several measurements of formation properties, in addition to downhole analysis of drilling parameters. These measurements are commonly made 30 to 50 ft behind the drill bit in special sections of the drill pipe. The data are analyzed by downhole computers, and because the drill pipe sections do not form a usable continuous electrical circuit, the final results are transmitted to the surface by pressure pulses in the mud column. The array of formation measurements that is available while drilling is now approaching the set of standard wireline data that are collected after the drill string is removed, and this trend will probably continue (Cantrell

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and others, 1992). Therefore, it is useful to consider wireline data as an analogue of what may be done in the future by a smart drilling system. Different types of measurements may be made by using sondes (i.e., instrument packages) suspended on a wireline cable after the drill string has been removed (Figure 6.1). The physics of these instruments is reviewed by Tittman (1986) and Ellis (1987), and an insight into geological interpretation is given by Doveton (1986). The measurements are conveniently grouped into electromagnetic, nuclear, acoustic, and mechanical categories. Usually the instrumentation consists of a source device, a receiver, and electronics to digitize and transmit the data to the surface. For measurements of some properties, such as the Earth's naturally occurring radioactivity, no source is required. FIGURE 6.1 Generalized logging setup (Desbrandes, 1982).

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Electromagnetic devices are used to distinguish resistive oil and gas from conductive brines in rock pore spaces. An example of an electromagnetic device is shown in Figure 6.2. Most rock is also resistive, however, and the overall resistivity depends on the porosity, that is, the volume of pore space as well as the conductivity of the enclosed fluids. The spontaneous potential log is typically used to estimate brine conductivity. Porosity can be estimated concurrently with conductivity by using nuclear and acoustic methods. The velocity of sound depends largely on the porosity and secondarily on the rock type. The scattering of neutrons or high-energy gamma rays from a source to a detector depends primarily on the porosity and secondarily on the rock type, and sondes have been constructed based on both properties. FIGURE 6.2 (a) Electrodes and current distribution for the Dual Laterolog in an infinite and homogeneous isotropic medium. The sonde is split for purposes of illustration. (b) Arrangement of electrodes on the sonde (Suau and others, 1972).

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Rock type is determined from the combination of porosity-sensitive measurements and chemical composition from natural or induced gamma rays. Additional, more specialized measurements are available to aid in the determination of sedimentary structures, fractures, faults, and pore-size distributions. Ultimately, the information available from wireline sondes enables a comprehensive characterization of the rock as well as the type, amount, and ultimate recovery of hydrocarbons. Although not as extensive as the available suite of wireline measurements, the set of measurements made while drilling is impressive (Gianzero and others, 1985; Jan and Harrel, 1987; Norve and Saether, 1989; Wraight and others, 1989). These measurements include parameters related to lithology, porosity, and hydrocarbon identification. Downhole formation measurements are being combined with one another, creating new opportunities in drilling efficiency (Betts and others, 1990). Measured formation properties can now be compared continuously to values expected for the target horizon, as long as the drill remains in the target formation, the sets of parameters agree, and the drilling trajectory is unaltered. However, when measured parameters deviate from expected values, it is likely that the drill is no longer in the target formation. The drilling trajectory can then be modified to remain within the target or to reenter the target zone. Recent advances in these technologies show that even more efficient drilling practices could be achieved if the intelligence of the drilling system is increased through the use of advanced sensing systems. Status of the Field In what follows the major components and principal problem areas of sensing systems are considered. The major components are bit-rock interaction; formation evaluation and fracture detection at and ahead of the bit; measurement of stress, rock strength, and pore pressure; drill pipe-rock interaction; position, direction, and steering analysis; cutting and mud analysis; environmental logging; telemetry; and data analysis, measurement interpretation, and activation systems.

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Bit-Rock Interaction Current measurements at the critical bit-rock interface are limited to the rate of penetration, the weight on the bit, the torque, and the rate of rotation. This set of measurements is also used to estimate bit wear (Fay, 1993). It is possible that these bit-rock interface measurements can be significantly enhanced through new discoveries in the fundamental physics and chemistry of bit-rock interaction. Advances now allow measurements at the bit that can establish the particular rock breaking mechanism that is active at that moment (Roy and Cooper, 1993). Other advances now allow monitoring of the state of wear of the bit from bit-emplaced sensors. Information gained from such monitoring indicates that the bit being used is not the most efficient under current conditions and that bit replacement, change in operating conditions, or other remedial action is called for. One recent development in this area is the Ocean Drilling Program's development of a cutter head with retractable bits that can be changed without removing the cutting head. Formation Evaluation and Fracture Detection at and Ahead of the Bit The measurement of physical and chemical (e.g., contaminants) properties at the drill bit and the prediction of rock properties ahead of the bit hold enormous potential. A great deal of progress has been made over the past decade in measurement-while-drilling (MWD) technology. Before MWD, one had to wait until the drill string was removed to log or directly observe formations for their lithology and mineral or hydrocarbon content. The time lag sometimes lasted several days. With MWD, sensing systems are placed in the drill string above the bit. Information about the formations encountered uphole is recorded and transmitted with the drill string in place hours after drilling. Current MWD systems enable sophisticated formation evaluation a few meters behind the current bit location by measuring a wide array of formation properties including resistivity, natural gamma-ray activity, bulk density, and neutron porosity. It is likely that in the near future the set of MWD measurements will expand to include most routine wireline logging data. Advances in MWD now reduce the distance lag behind the bit where measurements are currently made and expand the scope to include

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measurements ahead of the bit. Such measurements would hold great promise for many industries and applications. For example, in environmental logging, fractures carry contaminants into or from sensitive areas. In tunneling, open, water-bearing fractures cause enormous problems. Not only are the tunnelers immediately endangered, but the remedial cost is significant. It would be extremely beneficial to be able to detect and plan for such conditions before they are encountered by the bit. Another example involves oil and gas drilling where high-pressure gas kicks can ''blow out" a well. If such high-pressure conditions could be detected ahead of the drill there would be significant economic and safety benefit. Remote detection of such features may involve acoustic or electromagnetic probing. Such advances would have a rapid and profound impact on the combined drilling industries. Stress, Rock Strength, and Pore Pressure Stress, rock strength, and pore pressure help determine the degree of difficulty that a given bit at a given drilling state will encounter. These formation attributes are also key in the detection of overpressuring and possible blowout risks. Currently, estimates of the state of stress and rock strength are often difficult and expensive to obtain, and may require complicated procedures such as fracturing the formation or slow leak-off tests (Kunze and Steiger, 1992). A promising new tool to measure borehole deformation and fracture under pressure has recently been developed (Despax and others, 1989; Kuhlman and others, 1993); this development is regarded as an area of great potential for innovative new measurements and analysis. Drill Pipe-Rock Interaction Drill pipe interaction with the rock is an important area for evaluation. Drilling loses efficiency as energy is spent in friction with the rock. In addition, drill pipe-rock interaction decreases wellbore stability, resulting in reduced efficiency and, at worst, a stuck drill pipe or the need to abandon the hole (Steiger and Leung, 1990; Gibson and Tayler, 1992). Currently, no sensing system exists to identify depths and friction magnitudes. The state of stress, which is also not currently measured, is

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one of the most important parameters governing wellbore stability (Hill and others, 1992). For example, if it were known that the friction was concentrated at a depth of weak and friable shale and that a stronger unit lay slightly deeper, the drilling system parameters might be altered to minimize wellbore friction until the weak lithology was passed. Position and Direction Analysis Oil and gas wells are increasingly being drilled horizontally or at great inclinations from vertical. Horizontal drilling also is prevalent in the field of environmental remediation (Kaback and others, 1989). Although horizontal drilling is more expensive than conventional vertical drilling, it has the advantage of a greater length of the wellbore being in direct contact with the formation. For environmental remediation applications, this means that, in general, a greater fraction of a subsurface volume can be reached for remediation. In oil and gas wells, the length of the wellbore adjacent to quantities of oil and/or gas is dramatically increased, often by as much as a factor of 100. High-resolution, three-dimensional seismic surveys help to integrate directional drilling and the subsurface visualization that is provided by seismic data. Horizontal and directional drilling requires accuracy in the determination of bit location and in visualization of the drilling direction in three dimensions. Although some sensing systems exist in this domain (Gaudio and Beasley, 1991; Stephenson and Wilson, 1992; Tarr and others, 1992), major advances in directional drilling will require increased accuracy in these systems. Cuttings and Mud Analysis In terms of its composition and properties, the mud column (i.e., the vertical column of drilling mud in the borehole) is a dynamic system whose characteristics are frequently changing dramatically in both time and space. The mud composition changes as shales slough into the column and are dispersed into the mud, or by chemical interaction between the mud and the formation. Mud fluids are commonly filtered out of the column by the formation of a mudcake along the borehole. These mud fluids flow into the pore

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space and may interact with minerals in the formation. Compositional changes and varying temperature conditions in the mud column alter its rheological properties; these changes are not monitored. Sensing systems that monitor mud composition and rheological properties downhole, as well as at the surface, could lead to intelligent decisions about remedial mud modification and thus to more efficient drilling. Environmental Logging A new area of drilling advancement concerns logging in wells that are drilled for environmental evaluation or remediation. Environmental logging and measurement can identify and quantify hazardous materials, such as radioactive materials and dense nonaqueous phase liquids. Conventional logging has focused on measurements most applicable to oil and gas exploration and development. Although some of these techniques may be applicable to environmental logging, it is likely that significantly advanced systems for detection and monitoring will be needed that can respond to contaminant concentrations at levels of parts per million or parts per billion. For example, high-purity germanium gamma-ray detectors and some of the newer rare-earth orthosilicate detectors might be readily adaptable for radioactive element detection and monitoring. Fluid and rock sampling for uphole analysis is another need in environmental remediation. The potential exists for interactions among the remediation industry, the logging industry, and the national laboratories for applied research in this area. Environmental remediation itself can benefit from advances in drilling (Kaback and others, 1989). Novel drilling techniques, such as short turning radius, might be combined with innovative remediation systems to improve the efficiency of current practices dramatically. Telemetry A mud-pulse telemetry system is used to convey to the surface, measurements of the bit-rock interaction, formation properties, state of stress, and direction and orientation of the drill (Figure 6.3). Because coaxial cables or other data transmission devices are not viable in today's drilling systems, only the most vital information processed downhole can

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FIGURE 6.3 Downhole logging apparatus (Arps and Arps, 1964). be sent uphole at the slow transmission rate of a few bits per second. This slow transmission rate is currently a major limitation and will become a greater liability as more measurements are made in future drilling systems. Data Analysis and Activation Systems Measurements made downhole or in tunneling, mining, and environmental systems require action by the driller. For example, the trajectory may need modification, the intended target may have been reached, or a different trajectory may be needed until new data dictate otherwise. This

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process of data analysis is currently performed uphole by humans. The analysis involves comparison of data with external standards, as well as comparison of data generated by a forward model of what is expected according to preexisting concepts of the subsurface. It is possible that much of the decision-making capabilities might eventually reside in a downhole expert system. This would alleviate the need for increased data transmission to an uphole operator. Priorities for R&D R&D is needed to produce advanced, intelligent drilling systems that incorporate advanced sensors and microprocessors. The committee has identified eight specific areas for development: Bit-rock interaction: A greater understanding of the fundamental physics and chemistry of the bit-rock interaction will aid in advancing downhole sensing capabilities. Two particular areas of research that can be accomplished in the short term are recommended: (a) development of devices to sense the condition of the bit in order to warn of excessive wear or the need for replacement; and (b) development of sensors for the mud properties at the bit and throughout the wellbore to improve borehole quality and mud conditioning. Longer-term research should seek to develop a means of sensing the rock breakage mechanism to aid in bit selection strategies. Formation evaluation and fracture detection at and ahead of the bit : Although significant advances in MWD technologies have occurred during the last decade, great need remains for improved capabilities to make measurements around and ahead of the bit. Three long-term research areas are recommended: (a) development of fracture-detection capabilities based on electromagnetic, acoustic, and other innovative methods such as instantaneous mud loss; (b) development of advanced formation evaluation procedures based on sensor measurements; and (c) development of high-resolution imaging capabilities to aid in formation evaluation and steering decisions. Stress, rock strength, and pore pressure: Advances in sensing technologies will lead to an improved ability to measure in situ stress, rock 

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    strength, and pore pressure. Two long-term research efforts in this area are recommended: (a) the development of novel direct sensing of stress, rock strength, and pore pressure for optimizing drilling parameters, steering, minimizing formation damage, and formation evaluation; and (b) the development of technologies to predict pore pressure from acoustic measurements. Drill pipe-rock interaction: An improved understanding and means of evaluating drill pipe interaction with rock can lead to greater drilling efficiencies and increased wellbore stability. Two short-term research areas are recommended: (a) the development of sensors to detect, locate, and measure friction between the drill string and the formation to optimize drilling parameters and enhance wellbore stability; and (b) the development of sensors to determine drill pipe wear. Position and direction analysis: Advances in directional drilling are tied to improved capabilities for determining bit location and position. Two long-term research efforts in this area are recommended: (a) the development of sensing systems capable of determining the bit position (depth and spatial coordinates) within an accuracy of 1 ft; and (b) the development of enhanced systems to determine bit direction and the resultant forces to optimize steering capabilities. Environmental logging: Environmental evaluation and remediation would benefit from improvements in sensing systems for drilling, especially detection and monitoring of contaminants, fluid and rock sampling, and novel drilling techniques for remediation purposes. In the short term, research should be supported to develop advanced methods of sampling Earth materials. Longer-term research needs include (a) the development of advanced sensors to detect environmentally hazardous materials to aid in remediation efforts; and (b) the development of innovative techniques and sensors to monitor contaminated sites on a long-term basis. Telemetry: Drilling data generated and recorded downhole need to be transmitted uphole in a speedy and efficient manner. Present telemetry systems are capable of transmitting data at rates of a few bits per second. The incorporation of advanced sensors into the drilling system requires much higher rates of data transmission. Consequently, telemetry 

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systems capable of transmitting data at rates of kilobits per second should be developed. Data analysis and activation systems: Downhole data analysis and activation systems may improve overall drilling operations by reducing the need for data transmission uphole for operator decision making. Long-term research is recommended to develop sophisticated software for downhole analysis of drilling data. References Arps, J. J., and Arps, J. L., 1964, The subsurface telemetry problem—a practical solution: Journal of Petroleum Technology, v. 16 (5), p. 487-493. Betts, P., Bliunt, C., Broman, B., Clark, Hibbard, L., Louis, A., and Oosthoek, P., 1990, Acquiring and interpreting logs in horizontal wells: Oilfield Review, v. 2 (3), p. 34-51. Cantrell, L. A., Paxson, K. B., Keyser, B. L., and Ball, S., 1992, Case histories of MWD as wireline replacement: An evolution of formation evaluation philosophy: Proceedings SPE Annual Technical Conference, Washington, D.C., Oct. 4-7, p. 115-130. Desbrandes, R., 1982, Diagraphies dans les Sondages: Paris, Editions Technip, 584 pp. Despax, D. M., Martin, J. P. A., and Gueret, F. C., 1989, Geomechanical Probe for a Drilling Well: U.S. Patent 4,800,753, 11 pp. Doveton, J. H., 1986, Log Analysis of Subsurface Geology: New York, John Wiley and Sons, 273 pp. Ellis, D. V., 1987, Well Logging for Earth Scientists: New York, Elsevier, 532 pp. Fay, H., 1993, Practical evaluation of rock-bit wear during drilling: SPE Drilling and Completion, v. 8 (2), p. 99-104. Gaudio, G. B., and Beasley, J. C., 1991, A comparison of MWD and wireline steering tool guidance systems in horizontal drilling: Proceedings SPE Annual Technical Conference, Dallas, Tex., Oct. 6-9, Richardson, Tex., p. 7-18. Gianzero, S., Chemali, R., Lin, Y., Su, S., and Foster, M., 1985, A new resistivity tool for measurement-while-drilling: Society of Professional

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Well Log Analysts 26th Annual Logging Symposium Proceedings, p. A1-22. Gibson, M. T., and Tayler, P. J., 1992, Optimal applications engineering and borehole stability analysis avoid differential sticking and lead to successful open hole completion of North Sea horizontal well: Proceedings SPE Annual Technical Conference, Washington, D.C., Oct. 4-7, p. 741-754. Hill, T. H., Seshadri, P. V., and Durhan, K. S., 1992, A unified approach to drillstem-failure prevention: SPE Drilling and Completion, v. 7 (4), p. 254-260. Jan, Y.-M., and Harrel, J. W., 1987, MWD directional-focused gamma ray—a new tool for formation evaluation and drilling control in horizontal wells: Society of Professional Well Log Analysts 28th Annual Logging Symposium Proceedings, p. A1-A17. Kaback, D. S., Looney, B. B., Corey, J. C., Wright, L. M., and Steele, J. L., 1989, Horizontal wells for in situ remediation of groundwater and soils: National Water Well Association Outdoor Action Conference, Orlando, Fla., May 22-25, 1989, 16 pp. Kuhlman, R. D., Heemstra, T. R., Ray, T. G., Lin, P., and Charlez, P. A., 1993, Field tests of downhole extensometer used to obtain formation in-situ stress data: SPE 25905, SPE Rocky Mountain Regional/Low Permeability Reservoirs Symposium, Denver, April 12-14, 1993, Richardson, Tex., SPE, p. 625-634. Kunze, K. R., and Steiger, R. P., 1992, Accurate in situ stress measurements during drilling operations: Proceedings SPE Annual Technical Conference, Washington, D.C., Oct. 4-7, p. 491-499. Norve, K. N., and Saether, H., 1989, Field experience using the full suite MWD-combination for reservoir logging and evaluation: Society of Professional Well Log Analysts 30th Annual Logging Symposium Proceedings, p. A1-A26. Onoe, Y., Yoder, D., and Lawrence, M., 1991, Improved drilling performance, efficiency, and operations using an advanced real-time information system for drilling: Proceedings SPE Annual Technical Conference, Dallas, Oct. 6-9, p. 335-344. Roy, S., and Cooper, G., 1993, Prevention of bit balling in shales—preliminary results: SPE Drilling and Completion, v. 8 (3), p. 195-200. Steiger, R. P., and Leung, P. K., 1990, Prediction of wellbore stability in shale formations at great depth: Proceedings ISRM International

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Symposium on Rock at Great Depth, Rotterdam, Balkema, v. 3, p. 1209-1218. Stephenson, M. A., and Wilson, H., 1992, Improving quality control of directional survey with continuous inertial navigation: SPE Drilling and Completion, v. 7 (2), p. 100-106. Suau, J., Grimaldi, P., Poupon, A., and Sounaite, P., 1972, The Dual Laterolog-Rxo Tool: SPE 4018, Society of Petroleum Engineers 47th Annual Fall Meeting, San Antonio, Tex., Oct. 8-11, Richardson, Tex., SPE, v. 21, p. 121-132. Tarr, B. A., Kuckes, A. F., and Ac, M. V., 1992, Use of new ranging tool to position a vertical well adjacent to an horizontal well: SPE Drilling and Completion, v. 7 (2), p. 93-99. Tittman, J., 1986, Geophysical Well Logging: Orlando, Academic Press, 175 pp. Wraight, P., Evans, M., Marienbach, E., Rhein-Knudsen, E., and Best, D., 1989, Combination format in density and neutron porosity measurements while drilling: Society of Professional Well Log Analysts 30th Annual Logging Symposium Proceedings, p. B1-B23.